Near-infrared absorbing coloring matters that exhibit absorption in a near-infrared region of 700 to 2,000 nm have been conventionally studied for use in a variety of industrial applications. For example, they have been used for: optical information recording media such as CD-R; thermal CTP; flash toner fixation; printing such as laser thermal recording, etc.; and the like, as well as heat shielding film, etc. Furthermore, characteristic features of selectively absorbing light in a specific wavelength region are utilized to apply for near-infrared cut filter used in PDP filters, etc., as well as films for plant growth regulation, and the like. In addition, by dissolving or dispersing a near-infrared absorbing coloring matter in a solvent to prepare an ink, the coloring matter can be used also as a near-infrared absorbing ink which is unrecognizable by visual observation but is readable only with a near-infrared radiation detector, etc.
Inks such as near-infrared absorbing inks are generally classified into aqueous type and organic solvent-base type in accordance with principal components of the solvent; however, in light of environmental issue and the like, aqueous inks have been strongly demanded, and thus development of a near-infrared absorbing coloring matter soluble in water has been desired.
Known near-infrared absorbing coloring matters include immonium coloring matters, diimmonium coloring matters, dithiol metal complex coloring matters, cyanine coloring matters, and the like, and in particular, naphthalocyanine coloring matters have been known as a highly durable near-infrared absorbing coloring matter.
Most well-known naphthalocyanine coloring matters are pigments insoluble in solvents, or coloring matters that are soluble only in organic solvents (for example, see Patent Document 1), and naphthalocyanine coloring matters that exhibit favorable solubility in water are scarcely known.
In Examples of Patent Documents 2 and 3, a water soluble naphthalocyanine coloring matter is obtained by sulfonating a naphthalocyanine coloring matter. Although this compound is soluble in water, storage stability is inferior when provided as an aqueous ink composition, thereby leading to unsatisfactory practical applicability.    Patent Document 1: Japanese Patent No. 2507786    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H2-167791    Patent Document 3: U.S. Pat. No. 7,122,076